Close friend is much changed (4)
I believe the answer is:
chum
'close friend' is the definition.
(I've seen this before)
'much changed' is the wordplay.
'changed' indicates anagramming the letters.
'much' is an anagram of 'CHUM'.
'is' is the link.
(Other definitions for chum that I've seen before include "Pal, mate" , "Good friend" , "Informally, a close friend" , "Salmon" , "Close friend (colloq.)" .)